A census block group (BG) is a cluster of census blocks having the same first digit of their four-digit identifying numbers within a census tract. For example, Block Group 3 (BG 3) within a census tract includes all blocks numbered from 3000 to 3999. Block groups generally contain between 600 and 3,000 people, with an optimum population size of 1,500. Block groups never cross the boundaries of States, Counties or census tracts.